In order to observe the muscular action of the heart it is well known to attach electrodes to the body of a patient so as to obtain a continuous electric signal representing the muscular activity of the heart. When this signal is plotted or displayed relative to a linear time base there is provided an ECG waveform having certain characteristic points identified as P, Q, R, S and T. The Q, R, S portion of the ECG waveform is known as the QRS complex and its shape is indicative of normal or abnormal cardiac behavior.
Certain types of abnormal cardiac behavior indicate an imminent threat to the life of the patient and include premature ventricular contraction, commonly known as PVC. PVCs are made manifest by an irregularity in the QRS complex. The frequency at which PVCs occur is important since, for example, while one or two irregular PVC beats per minute may not present any immediate danger, five or more per minute indicates that the heart is likely to go into fibrillation at any time. While it is possible to visually observe an ECG waveform and detect the presence of PVCs, personnel having the necessary training and skill are not generally available. Moreover, constant personal observation of the waveform is both tedious and expensive.
There is disclosed in U.S. Pat. No. 3,828,768 a system for differentiating between a QRS complex corresponding to normal heart action and a QRS complex corresponding to abnormal heart action of the types indicating an immediate threat to the life of the patient. In the system disclosed in the said patent, the ECG signal from the patient is applied to high and low bandpass filters to separate the high and low frequency components of the signal. For a period of time including the QRS interval, the output from the low pass filter is gated to an integrator from which the integral is coupled to a comparator for comparison thereof with a manually set reference standard. An alarm is triggered and a PVC recorded each time the integral of the low frequency components of the QRS complex exceeds the reference standard.
While that patented system functions satisfactorily under certain controlled conditions, it is sensitive to extraneous muscular activities and movements of the patient which cause irregularities in the ECG waveform of the type generally known as artifacts. Also, the manner in which the length of the gating signal is determined permits improper triggering of the integrator. Consequently, false PVC detections are common with the system disclosed in the said patent.